This invention is related to highly localized Infrared (IR) spectra on a sample surface utilizing an Atomic Force Microscope (AFM) and a variable wavelength pulsed IR source, and specifically to dynamic IR power control for maximizing dynamic range while minimizing sample damage.
IR spectroscopy is a useful tool in many analytical fields such as polymer science and biology. Conventional IR spectroscopy and microscopy, however, have resolution on the scale of many microns, limited by optical diffraction. It would be particularly useful to perform IR spectroscopy on a highly localized scale, on the order of biological organelles or smaller, at various points on a sample surface. Such a capability would provide information about the composition of the sample, such as location of different materials or molecular structures. Conventional infrared spectroscopy is a widely used technique to measure the characteristics of material. In many cases the unique signatures of IR spectra can be used to identify unknown material. Conventional IR spectroscopy is performed on bulk samples which gives compositional information but not structural information. Infrared Microscopy allows collection of IR spectra with resolution on the scale of many microns resolution. Near-field scanning optical microscopy (NSOM) has been applied to some degree in infrared spectroscopy and imaging. Recently, a technique has been developed based on use of an AFM in a unique fashion to produce such localized spectra. This work was described in a publication entitled “Local Infrared Microspectroscopy with Sub-wavelength Spatial Resolution with an Atomic Force Microscope Tip Used as a Photo-thermal Sensor” Optics Letters, Vo. 30, No. 18, Sep. 5, 2005. This technique is also described in detail in co-pending applications U.S. Ser. Nos. 11/803,421 and 12/315,859, commonly owned by the assignee of this invention, and whose contents are incorporated by reference. Those skilled in the art will comprehend the details of the technique in the publication and patent applications but the technique will be described briefly herein for clarity. The general technique is also referred to as Photo-Thermal Induced Resonance, or PTIR.
Referring to FIG. 1, the PTIR technique basically uses an Atomic Force Microscope (AFM). A typical AFM cantilever probe 2 is brought in interaction with a region of a sample 1. A beam from pulsed IR radiation source 3 is directed to the sample 1. When a brief, intense radiation pulse illuminates the sample 1, it causes a rapid sample expansion due to thermal shock, stimulating a resonant oscillation 4 of the cantilever probe 2, which is measurable by the AFM's probe deflection detection system. The amplitude of the thermal shock depends on the degree of IR absorption, which will depend on the material characteristics of the sample in the area immediately under the probe tip. The degree of absorption will also depend on the wavelength of the IR radiation. Thus varying the wavelength of the source and repeating the deflection measurement across a range of wavelengths yields an absorption spectrum 5 for a highly localized region of a sample. The measurement may be repeated at a plurality of points on the sample surface, to create an absorption spectra map, enabling characterization and identification of sample material composition at a previously unattainable resolution. Related techniques detect the local temperature change in the sample via a temperature sensing AFM probes, as described by Hammiche and others in the scientific literature.
It has become apparent during applications testing of a commercial nanoscale IR spectroscopy platform there is a challenge between optimizing the signal to noise ratio and the risk of sample damage. The issue is related to the dynamic range of the measurement technique. If the amount of IR energy absorbed is small in some areas of the sample at particular wavelengths, it creates a signal that is below the limit of detection. To increase the signal to noise ratio, the IR laser power can be turned up to increase the amount of absorbed radiation. But if the absorbed IR energy is too high, it causes substantial heating of the sample in other areas and/or wavelengths which can lead to melting, burning and/or chemical changes in the sample. Even at temperatures below a thermal damage threshold, the sample may soften to the point that the pressure of an AFM tip can cause local plastic deformation, altering the topography of the sample. It is desirable to avoid any or all of these potential types of sample damage.
The temperature rise in the sample is a function of both the laser power at a given wavelength and the sample absorption at that wavelength, as illustrated in FIG. 2. A suitable radiation source using an optical parametric oscillator (OPO) laser has power variations of almost 20× over the range of 1000-2000 cm−1. In addition to this, sample absorption peaks can vary by orders of magnitude. In recently obtained spectra, the ratio between the largest and smallest peak heights can be 20×. So even over the range of 1000-2000 cm−1 the sample temperature change could vary by 20×20=400× over peaks of interest. If the laser power is turned up large enough to resolve the smallest peaks, the sample can easily be damaged at the higher peaks. (Note that the x-axes in FIG. 2 are labeled in “wavenumbers (cm−1)” a convention used in spectroscopy. The wavelength in microns is given by 10,000/wavenumber. In this application we use the terms wavenumber and wavelength interchangeably, i.e. measuring a property as a function of wavelength provides equivalent information as measuring a property as a function of wavenumber.)
A typical approach used in the PTIR technique is to use a lens to focus the infrared laser beam onto a region of the sample under the cantilever probe tip. It has also become apparent in testing by the inventors that it can be quite a challenge to determine the optimal alignment of an infrared laser beam and that this alignment process can require tedious experimentation or trial and error to find the best alignment. There are a number of reasons for this. First, the infrared laser beam is invisible to the eye and also to many video cameras. While IR cameras are available they are quite expensive. A common solution is to employ a visible guide beam that is collinear with the infrared beam. This is an imperfect solution, however, especially when the beams pass through refractive optics. The index of refraction of most optical materials has a significant dependence on wavelength. And so going from the visible guide beam to an infrared source, especially a widely tunable infrared source can create significant changes in the amount of refraction caused by a lens or other optical element. This change in refraction can lead to differences in the position of the focus spot created by a lens, particularly when using a monochromatic visible beam as an alignment reference for a broadly tunable infrared source. The differences in position can also be in three axes, one along the beam axis (focus depth) and two lateral axes perpendicular to the beam axis. While these shifts can be minimized by aligning the incident laser beam perpendicular to optical surfaces, in practice it can be difficult to achieve such alignment over a wavelength range, especially when multiple refractive elements are used and/or when one or more of the refractive elements are replaceable by the user (e.g., a sample substrate). However, despite these issues, it can be advantageous to utilize a visible alignment source as a mechanism for rough alignment. Further, tunable infrared sources may have a significant variation in exit beam angle themselves as a function of wavelength and these variations can also translate directly into lateral shifts in the focus position. Uncontrolled shifts in the focus and/or lateral position of the beam can cause loss of signal-to-noise, improper measurements of relative heights of absorption peaks, and/or discontinuities in spectra as the IR source is switched from one stage to another (for example switching from the signal to the idler in an OPO radiation source). This can also be a significant issue since the IR beam may be focused to a diffraction limited spot on the scale of a few to a few 10's of microns. Very small angular shifts could otherwise move the point of this focused spot entirely away from the point of probe-sample interaction. For example, using a 25 mm focal length lens, a 1 mrad shift in the beam position with wavelength can lead to a 25 μm shift in the focused spot position. It is possible to defocus the laser beam to create a larger spot, but-this can compromise the signal-to-noise ratio, especially for weak absorptions and/or wavelengths where the laser source has limited power. This has led to the problem of some spectroscopic measurements being constrained in some cases to a single laser stage where the beam pointing errors are sufficiently small to be tolerable. Then it has been necessary for users to realign and/or refocus the IR beam onto the region of tip-sample interaction before obtaining spectra in a wavelength range corresponding to a different laser stage. In the case that a quantum cascade laser is used as a radiation source it may be necessary to use several laser heads to cover a broad fraction of the mid-IR. Each of the laser heads may also have a slightly different emission angle making it very difficult to maintain the focus spot at the point of tip-surface interaction over a broad wavelength range. It is therefore desirable to enable a system to allow broadband spectroscopic measurements with sweeps of more than 500 cm−1 and preferably more than 2000 cm−1 without manual readjustment by the user.